DNA methlation is the attachment of a methly group on the end of the original strand of DNA that is being copied. This is helpful when there is an error in the coding of DNA. For example if a base pair of nucleotides is A-C it is obviously wrong. Methylation is the way that the DNA repair mechanisms can tell if it is the A or the C that needs to be changed.

I read somewhere that methyl insufficiency and abnormal DNA methylation might play significant roles in the development of several pathologies including birth defects, cancer, diabetes, heart disease and neurological disorders. Why would that be the case?

There are 10 types of people in this world. Those who understand binary, and those who don't.

I have never heard that methyl insufficiency and abnormal DNA methylation might play significant roles in the development of several pathologies including birth defects, cancer, diabetes, heart disease and neurological disorder. Where have you heard that? I think that the new technology would all just use DNA to recreate tissues therefore bypassing the "curse" of abnormal DNA methlation.